This invention relates to a rolling bearing for supporting a rotary shaft of a disk memory device such as an optical disk or a magnetic disk, and more particularly to a rolling bearing for supporting a rotary shaft of a spindle motor or a swing arm in a disk memory device such as a compact disk, a hard disk drive (HDD), or a mass storage floppy disk drive.
FIG. 1 shows a support structure for supporting the rotary shaft of a typical hard disk drive (HDD for short). This support structure comprises a pair of ball bearings 6 for supporting a rotary shaft 3 rotatably supporting a magnetic disk 2 through a hub 1, and another pair of ball bearings 7 supporting a rotary shaft 5 of an arm 4 carrying at its tip a magnetic head (not shown).
The pair of ball bearings 6 supporting the rotary shaft 3 of the magnetic disk 2 have their inner rings 8 mounted to the rotary shaft 3 so as to rotate together, and their outer rings 9 fixedly fitted in a fixed tubular member 11 of a spindle motor 10. The rotary shaft 3 is fixed to the center of a dish-shaped rotary member 12, and is rotated by a spindle motor 10.
The magnetic disk 2 rotatably supported by this structure is rotated at a high speed by the motor 10 through the shaft 3, while the swing arm 4 is, as its name suggests, swung so that its head can read and write data into and out of the disk.
The arm 4 is supported at its end remote from the head on the top of the rotary shaft 5. The rotary shaft is rotated about its axis by an actuator (not shown) such as a coil to pivot the arm 4 a predetermined angle to move its magnetic head to a predetermined position. As the shaft 5 swings, the head can thus read and write magnetic data in the effective memory area of the magnetic disk 2.
Heretofore, high-precision small ball bearings (miniature ball bearings: JIS B 0104 "bearing having outer diameter of less than 9 mm") have been used to support rotary shafts of hard disk drive. Bearings with dimensional accuracy and rotation accuracy of JIS 5 or over have been used.
Such rolling bearings have a retainer having the shape of a crown as shown in FIG. 3A or 3B. They comprise an annular retainer body having on its top surface a plurality of pairs of claws 21 arranged circumferentially at a predetermined pitch, each adjacent pair having opposed curved surfaces, defining a rolling element retaining pocket 22. A flat surface 23 as a reference surface from which the claws 21 protrude is formed between adjacent pockets 22.
The retainer of FIG. 3B has a plate-shaped upright wall 24 formed on each flat surface 23 to prevent leakage of grease.
If semisolid lubricant such as grease is used for lubrication of such roller bearings having retainers, lubricating (or stirring) resistance will be produced according to the viscosity of the lubricant. This increases the torque required to turn the rotary shaft supported by the bearing, and also causes torque fluctuations.
To prevent this problem, unexamined Japanese patent publication 8-21450 proposes to make the retainer itself from a lubricative composition comprising a mixture of a polyolefin resin and a lubricating oil.
One problem of the retainer disclosed in the abovesaid publication is that since the lubricating oil is retained by the highly oil-absorptive polyolefin resin, oil scarcely exudes onto the surface of the retainer. It is also difficult to keep oil exuded uniformly at a constant rate over a long period of time even if the oil is dispersed uniformly in the resin.
An object of the present invention is to provide a rolling bearing for supporting a rotary shaft of a disk memory device, which includes a retainer having good lubricative properties and which can lower and stabilize the torque required to turn the rotary shaft so that the disk memory device will exhibit high optical or magnetic recording performance over a long period of time.